This invention relates to carbon steel screws and similar fasteners having selectively hardened portions to create desired properties and behavior, and a method for making the selectively hardened screws.
Stainless steel screws having selectively hardened regions are known from U.S. Pat. No. 3,376,780, issued to Tanczyn. Tanczyn discloses a stainless steel screw having selectively hardened screw flight crests and a selectively hardened head region for insertion of a screwdriver. These regions are harder than the remaining portions of the screw. The stainless steel screw has a carbon content not exceeding 0.20% by weight, a chromium content of 10-25% by weight, a nickel content of 5-20% by weight, a copper content of 1-5% by weight, and an aluminum content of 0.25-2.5% by weight. The hardening is accomplished by cold-working the stainless steel at about 700-900xc2x0 F., and by age-hardening at about 1050-1250xc2x0 F. The hardening is the greatest in the regions of the greatest cold-working.
U.S. Pat. No. 4,295,351, issued to Bjorklund et at., discloses a stainless steel screw whose flight crests have been selectively hardened. The selective hardening is achieved by aggressive cold-working of the precursor fastener blanks, at sub-zero temperatures, during formation of the threads. Another selectively hardened stainless steel screw is disclosed in U.S. Pat. No. 4,289,006 issued to Hallengren.
U.S. Pat. No. 2,229,565, issued to Hallowell Jr., discloses a socket screw whose head portion is selectively hardened. The head portion of the screw is rapidly heated by induction to an elevated temperature. The entire screw is then quenched, causing hardening of the heated portion. The resulting screw may have a Rockwell xe2x80x9cCxe2x80x9d hardness (xe2x80x9cRCxe2x80x9d) of about 48-50 in the head region, and a lower RC of about 30-35 in the remaining portions.
U.S. Pat. No. 5,755,542, issued to Janusz et al., discloses a screw having selectively hardened threads at a lower end of the screw shank, and a selectively hardened tip. U.S. Pat. No. 5,605,423, issued to Janusz, discloses a stud having selectively hardened threads at a lower end of the stud, and a selectively hardened tip.
Certain standard carbon steel screws (having a single slot in the head) and cross-recessed screws (having two slots in the head which cross each other) can only be exposed to a limited driving torque from a driving tool (e.g. screwdriver). When the head slots are exposed to excessive turning force, the slots become enlarged and damaged, so that the driving tool can no longer effectively engage the slots.
Consideration has been given to hardening the head portion of screws to strengthen the slots. However, the hardening can cause the head and upper shank portion to become excessively brittle, resulting in 1) the head breaking from the screw shaft when excessive turning force is applied, 2) hydrogen embrittlement if the screws are plated, and 3) head-popping caused by thermal expansion and contraction of the substrate(s) to which the screw is applied, which creates stress that cannot be relieved by screw elongation. Also, selective heating of the head portion to cause hardening can result in distortion of the screw when the entire screw (having a varying temperature profile) is exposed to a quenching fluid.
The present invention is directed to a selectively hardened carbon steel screw having a differential hardness profile within the head portion. A screw is provided having a head portion, a shank portion below the head, and a lower end portion or tip. The head portion has a top surface, a bottom surface, a center, an outer rim, and at least one slot in the center for engaging a driving tool. The invention also encompasses a carbon steel screw having a selectively hardened tip which facilitates initial penetration of the screw into a substrate.
In accordance with the invention, the head portion is selectively hardened in the center and at the top so that the center of the head portion near the top is harder than the bottom of the head portion and the adjacent screw shank. Put another way, the ridges and walls defining the slot are selectively hardened at the top to provide strength and hardness and reduce damage caused by a driving tool. Yet the bottom of the head portion and the adjacent shank remain relatively soft and pliable, so that the head portion does not break away from the shank when high torque or high stress, such as shear stress, is applied.
The invention also includes a method for selectively hardening the head portion at the center and near the top. A source of heat, which can be a flame jet, is applied directly to the top and center of the head portion, causing that region to reach a temperature above 1400xc2x0 F. The maximum temperature reached at the top and center of the head portion is higher than the temperature reached at the bottom of the head portion or adjacent portion of the screw shaft. Then, the screw can be differentially quenched to reduce or prevent distortion. Differential quenching can be accomplished by aiming a quenching fluid directly at the top center of the head portion, to achieve maximum quenching at the hottest region. The quench fluid can then be allowed to flow from the head portion to the remaining portions of the screw, where less quenching is wanted.
The invention includes a similar technique for selectively heat treating and quenching the tip of a screw, to cause localized hardening.